fix: Prevent stale page writes from recreating moved page directories

[afbora]

Description

This fixes a race condition where a stale page instance could still write to an old page path after the page had been moved during reordering.

When that happened, the write path could recreate the missing directory and produce ghost or duplicate page folders.

The fix adds a guard before writing page content. If the original root no longer exists, Kirby resolves the page again from the current app state. If the page was moved in the meantime, the write now fails instead of recreating the old path.

I've implemented the suggestion in my previous comment. I hope you like it:
#7964 (comment)

Changelog

✨ Enhancements

• Prevents ghost duplicate page folders caused by stale concurrent writes after page reordering Concurrent requests can create duplicates when reordering pages #7964

Docs

For review team

• Add lab and/or sandbox examples (wherever helpful)
• Add changes & docs to release notes draft in Notion